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United States Patent |
5,793,129
|
Parsoneault
,   et al.
|
August 11, 1998
|
Low distortion interference fits for spindle motor assembly
Abstract
For a precision spindle motor, an invention for reducing distortion
resulting from interference fits of a thrust plate to spindle motor shaft
and of a counter plate to a journal. A method of assembly according to the
invention includes the step of balancing an interference pressure about a
centerline of at least one of the thrust plate and the counter plate of
the spindle motor. A precision spindle motor according to the invention
comprises a journal having a bore defined therein, a shaft located in the
bore, a thrust plate mounted to a top portion of the shaft by means of an
interference fit, and a counter plate mounted to the journal adjacent to
the thrust plate by means of an interference fit, wherein an interference
pressure is balanced about a centerline of at least one of the thrust
plate and the counter plate.
Inventors:
|
Parsoneault; Norbert Steven (Watsonville, CA);
Leuthold; Hans (Santa Cruz, CA);
Jennings; David John (Santa Cruz, CA);
Murthy; Samnathan (Campbell, CA)
|
Assignee:
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Seagate Technology, Inc. (Scotts Valley, CA)
|
Appl. No.:
|
866810 |
Filed:
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May 30, 1997 |
Current U.S. Class: |
310/42; 29/525; 29/596; 310/67R |
Intern'l Class: |
H02K 015/14 |
Field of Search: |
310/42,67 R
360/78.13,104,106
29/525,596,598
|
References Cited
U.S. Patent Documents
4739427 | Apr., 1988 | Kilmer et al. | 360/98.
|
4828467 | May., 1989 | Brown | 418/201.
|
4920812 | May., 1990 | Okamoto et al. | 74/6.
|
5149111 | Sep., 1992 | Han | 279/2.
|
5313355 | May., 1994 | Hagen | 360/104.
|
5358339 | Oct., 1994 | Konno et al. | 384/107.
|
5666716 | Sep., 1997 | Parsoneault | 29/596.
|
Primary Examiner: Stephan; Steven L.
Assistant Examiner: Jones; Judson H.
Attorney, Agent or Firm: LaRiviere, Grubman & Payne
Parent Case Text
This is a continuation-in-part of application Ser. No. 08/397,566, filed
Mar. 2, 1995, now U.S. Pat. No. 5,666,716.
Claims
We claim:
1. A method of assembly of a precision spindle motor, the motor including a
journal and a counter plate, the method comprising the step of
interference fitting an outer side surface of the counter plate to an
inner interference surface of the journal, the outer side surface of the
counter plate defines a non-cylindrical shape for balancing an
interference pressure about the centerline of the counter plate.
2. A method of assembly of a precision spindle motor, the motor including a
journal and a counter plate, the method comprising the step of
interference fitting an outer side surface of the counter plate to an
inner interference surface of the journal, the inner interference surface
of the journal defines a non-cylindrical shape for balancing an
interference pressure about the centerline of the counter plate.
3. A method of assembly of a precision spindle motor, the motor including a
journal and a counter plate, the method comprising the step of
interference fitting an outer side surface of the counter plate to an
inner interference surface of the journal, the inner interference surface
of the journal defines a taper for balancing an interference pressure
about the centerline of the counter plate.
4. A precision spindle motor comprising:
a journal having a bore defined therein, the journal including a tube-like
protrusion defining an inner interference surface;
a shaft located in and extending from the bore of the journal, a top
portion of the shaft extending from the bore, the top portion of the shaft
including an outer interference surface;
a thrust plate having a hole defined therein by an inner surface of the
thrust plate, the thrust plate being mounted to the top portion of the
shaft by means of an interference fit between the outer interference
surface of the shaft and the inner surface of the thrust plate;
a counter plate including an outer side surface, the counter plate being
mounted to the journal adjacent to the thrust plate by means of an
interference fit between the inner interference surface of the journal and
the outer side surface of the counter plate, wherein an interference
pressure is balanced about a centerline of the counter plate.
5. A precision spindle motor according to claim 4 wherein at least one of
the outer side surface of the counter plate and the inner interference
surface of the journal defines a non-cylindrical shape for balancing an
interference pressure about the centerline of the counter plate.
6. A precision spindle motor according to claim 5 wherein the inner
interference surface defines a taper for balancing an interference
pressure about a centerline of the counter plate.
Description
TECHNICAL FIELD
The invention relates to computer disk drives. More specifically, the
invention relates to an external stator, hydrodynamic spindle motor
assembly for a computer hard disk drive.
BACKGROUND ART
Hydrodynamic spindle motors manufactured for hard disk drives are typically
made for a confined volume. Consequently, very tight tolerances must be
observed. Small deformations owing to press (interference) fitting can
violate the requirements of these tight tolerances. Thus, a problem with
assembly of spindle motors was controlling the amount of deformation owing
to interference fits.
DISCLOSURE OF INVENTION
The present invention is directed at low distortion interference fits for a
spindle motor in a hard disk drive assembly. Particularly, the present
invention is directed at press fitting a thrust plate to a spindle motor
shaft and at press fitting a counter plate to a spindle motor journal for
an external stator, hydrodynamic spindle motor.
The present invention provides a method of assembly of a precision spindle
motor. The method comprises assembling a thrust plate around a top,
exposed portion of a spindle motor shaft. The thrust plate is attached to
the portion of the shaft by means of a press fit. To reduce axial
deformation of the thrust plate for maintaining a consistent thrust gap,
an undercut is made in the shaft. The undercut provides a relief for the
press fitting of the thrust plate to the spindle motor shaft for
equalizing interference pressure about a centerline of the thrust plate.
The undercut may be exactly specified for causing the net pressure from
the press fit to act at the centerline of the thrust plate. In other
words, the pressure above the centerline and the pressure below the
centerline are approximately to exactly equal. This equalization results
in improved perpendicularity, while balancing interference pressure about
the centerline of the thrust plate thus reducing warpage.
The present invention also provides a method of press fitting a counter
plate to a spindle motor journal. The counter plate is made with a chamfer
around its outer periphery. Because the counter plate is pressed into a
relatively thin walled tube-like portion of the journal, the walls forming
the tube-like portion tend to bend away from the counter plate, i.e., the
thin walls deform in a generally radial direction away from the counter
plate. The chamfer extends to the center of the counter plate. The chamfer
provides "line contact" with the journal at the centerline of the counter
plate. Because of the chamfer, interference pressure acts at the
centerline of the counter plate. Thus, there is approximately no net
moment which would normally cause the counter plate to warp. As with the
thrust plate, the interference pressure is effectively balanced about the
counter plate centerline.
More particular, according to the invention there is provided a method of
assembly of a precision spindle motor, the motor including a journal, a
counter plate, a shaft and a thrust plate, the thrust plate for assembly
to the shaft by means of an interference fit and the counter plate for
assembly to the journal by means of an interference fit, the method
comprising the step of balancing an interference pressure about a
centerline of at least one of the thrust plate and the counter plate.
Preferably, the balancing step comprises the step of interference fitting
an outer side surface of the counter plate to an inner interference
surface of the journal, at least one of the outer side surface and the
inner interference surface being shaped to balance an interference
pressure about the centerline of the counter plate.
Alternatively, the balancing step comprises the step of interference
fitting an inner surface of the thrust plate to an outer interference
surface of the shaft, at least one of the outer interference surface of
the shaft and the inner surface of the thrust plate being shaped to
balance an interference pressure a bout the centerline of the thrust
plate.
Further according to the invention there is provided a method of assembly
of a precision spindle motor, the motor including a journal and a counter
plate, the method comprising the step of interference fitting an outer
side surface of the counter plate to an inner interference surface of the
journal, at least one of the outer side surface of the counter plate and
the inner interference surface of the journal being shaped to balance an
interference pressure about the centerline of the counter plate.
Also according to the invention there is provided a method of assembly of a
precision spindle motor, the motor including a shaft and a thrust plate,
the method comprising the step of interference fitting an inner surface of
the thrust plate to an outer interference surface of the shaft, at least
one of the outer interference surface of the shaft and the inner surface
of the thrust plate being shaped to balance an interference pressure about
the centerline of the thrust plate.
The invention also provides a precision spindle motor comprising:
a journal having a bore defined therein, the journal including a tube-like
protrusion defining an inner interference surface;
a shaft located in and extending from the bore of the journal, a top
portion of the shaft extending from the bore, the top portion of the shaft
including an outer interference surface;
a thrust plate having a hole defined therein by an inner surface of the
thrust plate, the thrust plate being mounted to the top portion of the
shaft by means of an interference fit between the outer interference
surface of the shaft and the inner surface of the thrust plate;
a counter plate including an outer side surface, the counter plate being
mounted to the journal adjacent to the thrust plate by means of an
interference fit between the inner interference surface of the journal and
the outer side surface of the counter plate, wherein
an interference pressure is balanced about a centerline of at least one of
the thrust plate and the counter plate.
As will be apparent to those of ordinary skill in the art of the present
invention, the present invention provides a spindle motor, and method of
assembling same, which is inherently insensitive (less warpage) to the
amount of interference, which allows for larger tolerances on parts thus
reducing cost.
Other features of the present invention are disclosed or apparent in the
section entitled: "BEST MODE FOR CARRYING OUT THE PRESENT INVENTION."
BRIEF DESCRIPTION OF DRAWINGS
For fuller understanding of the present invention, reference is made to the
accompanying drawing in the following detailed description of the Best
Mode of Carrying Out the Present Invention. In the drawings:
FIG. 1 is a cross section of a portion of a spindle motor in accordance
with the present invention;
FIG. 2 is an enlarged view of a circled area of FIG. 1;
FIG. 2A is a bottom elevation view of a counter plate in accordance with
the present invention;
FIG. 2B is a side plan view of a counter plate in accordance with the
present invention;
FIG. 3 is an enlarged view of a circled area of FIG. 1;
FIG. 4 is a top elevation view of a journal in accordance with the present
invention;
FIG. 5 is a side plan view of a counter plate in accordance with the
present invention; and
FIG. 6 is a side plan view of a thrust plate in accordance with the present
invention;
FIG. 7 is a cross sectional partial view of a journal and hub/sleeve
assembly of an alternative embodiment of a spindle motor in accordance
with the invention;
FIG. 8 shows the journal of FIG. 7 with a counter plate assembled thereto;
FIG. 9 illustrates the interference pressure on the counter plate
illustrated in FIG. 8.
Reference numbers refer to the same or equivalent parts of the present
invention throughout the several figures of the drawing.
BEST MODE FOR CARRYING OUT THE PRESENT INVENTION
As hard disk drives and spindle motors are both well known in the art, in
order to avoid confusion while enabling those skilled in the art to
practice the claimed invention, this specification omits many details with
respect to those known items.
Referring to FIG. 1, there is shown a cross section of spindle motor 10 in
accordance with the present invention. Spindle motor 10 is for a computer
hard disk drive and comprises base 11, magnet 12, stator assembly 13,
shaft 14, thrust plate 15, counter plate 16, o-ring 17, journal 18, and
hub/sleeve assembly 19. Spindle motor 10 is particularly well suited for a
fluid dynamic bearing application.
Referring now to FIG. 2, there is shown an enlarged view of circled area 20
of the cross-sectional view of FIG. 1. As shown, counter plate 16 is made
having a chamfer 21 in accordance with the present invention. As shown in
the bottom elevation view of counter plate 16 in FIG. 2A, chamfer 21
extends around counter plate 16. In the side plan view of counter plate 16
in FIG. 2B, centerline 23 indicates the extension of chamfer 21 to center
22. By extending to center 22, chamfer 21 facilitates "line contact" at
centerline 23 as between counter plate 16 and journal 18 when assembled to
journal 18.
Protrusion 33 forms a tube-like contour for receiving counter plate 16. The
tube-like contour of protrusion 33 of journal 18 is relatively thin as
compared with counter plate 16.
In continued reference to FIG. 2 in combination with FIG. 2B, assembly of
counter plate 16 to journal 18 results in interference fit 43.
Interference fit 43 causes protrusion 33 to deflect radially away from
counter plate 16 and into region 42. Without chamfer 21, counter plate 16
would bow in an axial direction due to pressure from interference fit 43.
However, owing to line contact, as facilitated by chamfer 21, between
counter plate 16 and protrusion 33 of journal 18, interference pressure
between counter plate 16 and protrusion 33 acts at centerline 23 of
counter plate 16. This balancing of interference pressure about centerline
23 results in approximately no net moment for inducing counter plate 16
warpage in an axial direction. Consequently, top thrust gap 49 (shown in
FIG. 3) requirements are not violated.
Referring to FIG. 3, there is shown an enlarged view of circled area 30 of
the cross-sectional view of FIG. 1. Shaft 14 is made having a relief 31 in
accordance with the present invention. Relief 31 extends around shaft 14
as shown in FIG. 1. Thrust plate 15 is mated with shaft 14 via press fit
37. Reservoir region 9, shown in FIG. 3, comprises two narrow gaps,
namely, bottom thrust gap 48 and top thrust gap 49. Reservoir region 9 is
particularly well suited for holding lubricating fluid (e.g., oil) for a
fluid dynamic bearing application.
Normally, press fit 37 would cause deformation of thrust plate 15 violating
thrust gap 48, 49 requirements. However, relief 31 is geometrically shaped
balancing interference pressure distribution about centerline 50 of thrust
plate 15 such that axial translation of thrust plate 15 is reduced. Relief
31 is shaped for minimizing axial translation of thrust plate 15, by
causing the pressure to be approximately equally distributed about
centerline 50 of thrust plate 15. As interference pressure owing to press
fit 37 is balanced about centerline 50, approximately no net moment
results for causing warpage of thrust plate 15 in an axial direction. It
will be appreciated that, in the illustrated embodiment, the balancing of
the interference pressure is obtained by providing a contact area between
the outer interference surface of the shaft 14 and the inner surface of
the thrust plate 15 which is asymmetrically distributed about the
centerline of the thrust plate.
While the actual resultant shape of relief 31 will depend in part on the
geometries involved, the method of the present invention reduces warpage
of thrust plate 15 such that thrust gap 48, 49 requirements are not
violated.
With reference to FIGS. 1 and 3 in combination, by control of the
geometries, the present invention reduces warpage owing to interference
fitting counter plate 16 and thrust plate 15 to form spindle motor 10.
Thus, with the present invention, substantially uniform and flat widths 47
for thrust gaps 48, 49 may be achieved, i.e., thrust plate 15 and counter
plate 16 are ostensibly longitudinally flat after assembly.
Referring to FIG. 4, there is shown a top elevation view of journal 18 in
accordance with the present invention. Referring to FIG. 1 in combination
with FIG. 3B, journal 18 comprises protrusions 33 and 35. Protrusions 33
and 35 in combination define gap 34. Gap 34 is for receiving o-ring 17.
Protrusion 33 defines a gap for receiving counter plate 16, as shown in
FIG. 1. Protrusion 35 defines a gap 36 for receiving a portion of shaft 14
and for receiving thrust plate 15. Shaft 14 is received through opening 32
of journal 18. Protrusion 35 defines a boundary of a reservoir region 9
(shown in FIG. 3).
Referring to FIG. 5, there is shown a side plan view of counter plate 16.
Without chamfer 21, counter plate 16 would likely bow in an axial
direction 45 owing to interference pressure, as indicated by phantom
counter plate 16a. Consequently, top thrust gap 49 would be increased,
especially in central region 53.
Referring to FIG. 6, there is shown a side plan view of thrust plate 15.
With continuing reference to FIG. 6 in combination with reference to FIG.
3, without relief 31, thrust plate 15 would likely warp at each end in
axial direction 45, as indicated by phantom thrust plate 15a.
Consequently, top thrust gap 49 would be narrowed to obliterated,
especially at distal regions 52, and bottom thrust gap 48 would be
increased, especially at distal regions 51. The amount of warpage of
thrust plate 15a will vary owing to the specific geometries of the
assembled parts. For example, the extent of shoulder 46 of shaft 14 likely
will affect the amount of warpage of thrust plate 15a. Thus, it is
necessary that relief 31 be formed to an appropriate extent bordering
thrust plate 15 to achieve balanced distribution of interference pressure
about centerline 50.
Referring to FIGS. 1 and 3 in combination, motor 10 stiffness is dependent
in part on maintaining thrust gaps 48, 49. By maintaining thus gaps 48,
49, minimum widths for those gaps are provided. However, it is further
important that gaps 48, 49 be substantially uniform, i.e., maximum and
minimum width tolerances must be maintained and warpage must be minimized
to maintain flatness. Typically, thrust plate 15 and counter plate 16 are
made exceptionally flat, namely, flat to approximately one-half (1/2)
micron. If thrust gap 48 or 49 is narrowed, then motor 10 is more subject
to rubbing and/or seizing, i.e., very stiff. A motor 10 operating at
approximately 10,000 revolutions per minute (rpm) suddenly seizing is not
desirable. Moreover, owing to the operation of motor 10, if thrust gap 48
or 49 is widened, then motor 10 is less stiff, resulting in loss of load
carrying capability. Therefore, it should be appreciated that even slight
deformations of counter plate 16 and thrust plate 15 may affect
performance of motor 10, especially where thrust gaps 48 and 49 are very
narrow, namely, typically on the order of seven (7) microns wide. For
thrust gaps which are especially narrow, approximately a five percent (5%)
tolerance for each thrust gap 48, 49 is typical. (Notably, while specific
values are stated herein, it should be understood that these values, e.g.,
speed of 10,000 rpm, flat within 1/2 micron, width of 7 microns, and
tolerance of 5%, are merely illustrative of one embodiment of the present
invention; it should be understood that the present invention may be
practiced with other values for different embodiments.)
A further embodiment of the invention is illustrated in FIGS. 7 to 9. FIG.
7 illustrates a portion, corresponding to the circled area 20 in the FIG.
1 embodiment, of a journal 100 without a counter plate assembled thereto,
while FIG. 8 illustrates the same portion of the journal 100 with a
counter plate 102 assembled thereto.
In this embodiment of the invention, the structure and configuration of all
of the components of the spindle motor, with the exception of the journal
100 and the counter plate 102, remains unchanged from the first
embodiment. In particular, the balancing of an interference pressure about
the centerline of the thrust plate 15 by providing a relief 31 in the
shaft 14, is unchanged. However, in this embodiment, the journal 100 is
different from the journal 18, and the counter plate 102 is different from
the counter plate 16.
Referring to FIG. 7, the journal 100 has a tube like protrusion 104 which
has an inner interference surface 106 to which the counter plate 102 is
fitted. As can be seen from the figure, the inner interference surface is
non-cylindrical in shape, and in fact defines a conical taper. As in the
previous embodiment, the journal 100 also has a recess 108 defined therein
for accommodating an O-ring 110.
The counter plate 102 in this embodiment is cylindrical in shape and does
not have any chamfer or relief 21 as in the previous embodiment. The
counter plate 102 defines an outer side surface 112 which fits against the
inner interference surface 106 of the journal 100, to form an interference
fit as shown in FIG. 8.
As mentioned above, the interference fit between the inner interference
surface 106 of the journal 100 and the outer side surface 112 of the
counter plate 102 causes the protrusion 104 to deflect radially away from
counter plate 102. This happens because the protrusion 104 is unsupported
at its upper edge. This deflection could create an uneven force
distribution across the outer side surface of the counter plate, which
would cause the warping illustrated in, and described above with reference
to, FIG. 5.
By providing the inner interference surface 106 as a inwardly tapered
surface as shown, the deflection of the protrusion 104 after assembly is
anticipated and compensated for. In effect, upon assembly, the more
compliant unsupported upper edge of the protrusion 104 is forced to defect
further than the supported lower edge of the protrusion 104, thereby
creating the substantially uniform interference pressure across the outer
side surface of the counter plate 102, as illustrated in FIG. 9. This can
also be appreciated in FIG. 8 which illustrates the counter plate 102
interference fitted to the journal 100, and the resulting deflection of
the protrusion 104.
It will be appreciated that the taper of the surface 106 and the deflection
of the protrusion 104 has been exaggerated for purposes of illustration.
In practice, the inner surface 106 defines a frusto-conical surface with
an angle relative to the axial direction of the thrust plate of 2.degree..
Referring again to FIG. 9, it can be seen that the interference pressure is
balanced about the centerline 112 of the counter plate 102. Accordingly,
there is no net moment about the centerline 112, which might otherwise
cause the counter plate to warp. Accordingly, the uniformity and flatness
of the bearing gap defined between the counter plate 102 and the thrust
plate 15 is maintained.
While the embodiment of the invention described with reference to FIGS. 1
to 6 provides equally consistent thrust gaps when compared with the
embodiment of the invention described with reference to FIGS. 7 to 9,
Applicants consider the latter embodiment of the invention to be the "best
mode" of the invention. This is because the journal 100 already requires
machining steps in is manufacture, and the machining of the tapered
surface 106 is therefore easily included as a modification of the existing
machining processes. On the other hand, the counter plates 102 are
preferably stamped, and the creation of the chamfer 21 require an
additional, independent machining step.
It should be noted that, for the purposes of this specification, the term
"journal inner interference surface" shall mean the inner surface of the
journal directly opposite to the outer side surface of the counter plate
after assembly, and which, but for any profiling of either surface, would
be available to participate in the interference fit between the counter
plate and the journal. Similarly, the term "shaft outer interference
surface" shall mean the outer surface of the shaft directly opposite to
the inner surface of the thrust plate after assembly, and which, but for
any profiling of either surface, would be available to participate in the
interference fit between the thrust plate and the shaft. Therefore, in
FIG. 3, the outer interference surface of the shaft 14 includes both the
relief 31 and the portion of the shaft 14 actually participating in the
interference fit 37.
Consequently, the present invention provides method and apparatus for
assembly of a spindle motor with the above-described interference fits
while maintaining tight thrust gap tolerances. While the present invention
has been particularly shown and described with respect to certain
preferred embodiments and features thereof, it should be readily apparent
to those of ordinary skill in the art that various changes and
modifications in form and detail may be made without departing from the
spirit and scope of the invention.
In particular, it will be appreciated that, while in the FIG. 3 embodiment,
the shaft 14 defines a non-cylindrical surface for providing a contact
area between the shaft outer interference surface and the thrust plate
inner surface which is asymmetrically distributed about the center line of
the thrust plate, the inner surface of the thrust plate could be provided
with the non-cylindrical surface instead. Also, instead of the relief 31,
the shaft outer interference surface or the thrust plate inner surface
could be provided with a taper similar to that of the inner surface 106 of
the journal 100. Further, the journal inner interference surface or the
counter plate outer side surface could be provided with a relief wherein
the contact area between the outer side surface of the counter plate and
the inner interference surface of the journal after interference fitting
is asymmetrically distributed about the centerline of the counter plate,
for balancing the interference pressure about the centerline of the
counter plate.
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